1. Field of the Invention
This invention relates generally to electrical switch assemblies, and more particularly to improved switch assemblies comprising a plurality of switches in a predefined arrangement, and to methods for manufacturing such switch assemblies. The invention has particular utility in connection with keyswitches in a keyboard assembly and will be described in connection with such utility.
2. Description of the Prior Art
Electrical switch assemblies for sensing the activation of keys in a keyboard system and for producing signals representative of keyboard activations are well known in the art. In general, prior art keyboard switches comprise a circuit board having a plurality of discontinuous, conductive pathways, separate from one another, on one or both surfaces thereof, and a plurality of switching means for selectively, electrically connecting the pathways, carried on one surface of the board. Typically the switching means comprise individual contact elements supported in a plane remote from that of the circuit board, and spaced from the conductive pathways. Resilient spring means are provided for maintaining the contact elements in the remote plane so that the individual switches are normally open. For example, Robinson et al U.S. Pat. No. 4,018,999 disclose a keyboard switch assembly comprising a plurality of switches in a predefined arrangement formed on one face of an insulative circuit board, and having a protective insulative coating located over the entire circuit board to retain the switches on the board, and also to seal the switches. According to Robinson et al each switch has an annular outer contact and a concentric inner contact, and includes a resiliently deformable, dome-shaped activating element. The activating element has an annular marginal edge which is shaped to coincide with the outer contact and is positioned on top of the outer contact to protrude therefrom. In operation, a downward force is applied to the upwardly protruding portion of the actuating element so that the element is deformed downwardly and touches the inner contact, thereby completing an electrical circuit between the inner and outer contacts.
Another type keyboard switch is disclosed in Webb et al U.S. Pat. No. 3,653,038. In the Webb et al switch an insulation sheet of thin dielectric film is formed over the inner contacts. Accordingly, with the Webb et al switch when the activating member is deformed no direct conductive path is formed between the inner and outer contacts. Instead, deforming the activating member greatly increases the capacitance between the contacts, whereby a current surge, which is detectable by known means, is produced in a coupled associated circuit.
Still other variations of keyboard switches are disclosed in Wiener, U.S. Pat. No. 3,383,487; Sudduth, U.S. Pat. No. 3,699,294; Seeger, Jr. et al, U.S. Pat. No. 3,789,167; Lynn et al, U.S. Pat. No. 3,860,771; Flint et al, U.S. Pat. No. 4,083,100; Pounds, U.S. Pat. No. 4,042,439; Dunlap, U.S. Pat. No. 4,085,306; Pounds et al, U.S. Pat. 4,195,210; Satoh, U.S. Pat. No. 4,218,603; Kissner, U.S. Pat. No. 4,218,600; and, Johnson, U.S. Pat. No. 4,254,309.
Manufacturing keyboard switches of the above described prior art types is relatively expensive. Such prior art types require a number of parts which must be separately manufactured and inventoried. Also, accurate positioning of the various actuating members with respect to their associated contacts presents manufacturing problems. With respect to the positioning problems the art has proposed certain solutions; however, as reported by Flint et al in U.S. Pat. No. 4,083,100, existing solutions to such positioning problems either have proved too inaccurate, or have required complex and expensive equipment. For example, Robinson et al have suggested that the actuating members may be initially oriented with respect to the circuit board by means of an appropriate template. However, before the insulation sheet can be secured over the circuit board the template has to be removed. This presents a problem since the activating members have to be retained in place independently of the template once they are positioned with respect to the circuit board. In order to solve this latter problem Robinson et al propose assembling the actuating members to the circuit board on a specially designed transfer base. The latter is constructed to receive the circuit board and to retain the domes with respect thereto by means of an externally applied vacuum or by means of magnet members. As will be appreciated, the transfer base not only constitutes an additional complication and expense in the assembly process, but also introduces difficulties in accurate alignment, because the transfer base, the circuit board and the template all have to be aligned accurately. Also, when the template is removed, the actuating members are in an exposed position with respect to the circuit and although restrained with respect to the circuit board, could be damaged or accidentally moved out of position during assembly. Also, the insulation sheet necessarily is placed on top of the circuit board and actuating members prior to being secured thereto, and, in being moved about, could accidentally displace one or more actuating members. Moreover, with this technique the insulation sheet is supported only on the tops of the actuating members and is not held flat immediately prior to being secured to the board. Thus, creases or similr imperfections in the insulation sheet material near the perimeter of an actuating member might not be secured to the board and thereby would permit some movement of the actuating member. As a result of the actuating member could move with respect to the circuit board during separation, so that, even though accurately positioned initially, the actuating member could move out of position during use. The foregoing and other limitations of positioning accuracy, manufacturing convenience, switch reliability and manufacturing cost are believed inherent in many prior art keyboard switch assemblies and the manufacture of such switch assemblies. Moreover many prior art keyboards are believed to be prone to failure in the field due to mechanical and/or electrical breakdown. With regard to this latter problem, it has thus become a common practice in the art to plate switch contacting members with a non-corrosive metal such as gold. Such plating requirements add significantly to manufacturing costs. Moreover, the inter-connections may also be subject to degradation due to metal loss from the switch contacting members upon repeated mating and unmating interconnections.
It is thus a principal object of the present invention to provide a novel and improved electrical switch assembly which overcomes the aforesaid and other problems of the prior art. Yet other objects of the present invention are to provide highly reliable electrical switches, particularly for use in keyboard applications, and low-cost methods for producing electrical switches of the type above described. Still more specific objects are to provide novel and improved keyboard switches which are characterized by extreme reliability, and low-cost method for producing custom switch patterns.